1. Field of the Invention
The present invention relates to a bearing provided with a rotation sensor, and more specifically, it relates to the structure of a bearing employed for a general-purpose motor requiring a rotation detecting function.
The present invention also relates to a bearing provided with a rotation sensor, and more specifically, it relates to a method of extracting a signal from a bearing provided with a rotation sensor.
The present invention further relates to a bearing provided with a rotation sensor, and more specifically, it relates to a bearing provided with a rotation sensor having a function of detecting the number of rotations or a rotational direction.
The present invention further relates to a bearing provided with a rotation sensor and a motor employing the same, and more specifically, it relates to a bearing provided with a rotation sensor supporting a shaft requiring a rotation detecting function. More particularly, the present invention relates to a bearing provided with a rotation sensor used in the vicinity of a general-purpose motor or the like generating a large magnetic field.
2. Description of the Prior Art
(First Prior Art)
The structure of a bearing 500 provided with a rotation sensor according to first prior art is described with reference to FIG. 26. FIG. 26 is a sectional view showing the structure of the bearing 500 provided with a rotation sensor. This bearing 500 provided with a rotation sensor, forming an antifriction bearing, comprises an outer ring 1, an inner ring 3 and rolling elements 2. A shielding member is provided between the outer ring 1 and the inner ring 3.
When the inner ring 3 is employed as a rotating bearing ring, a pulser ring 4 is fixed to the inner ring 3 with a mandrel 5. When the outer ring 1 is employed as a fixed bearing ring, a magnetic sensor 8 is fixed to the outer ring 1 with a sensor case 7 and a sensor case fixing ring 6. The bearing 500 provided with a rotation sensor having the aforementioned structure, which is compact and strong with no requirement for assembly control, is applied to a support bearing for the rotary shaft of a motor.
(Problem of First Prior Art)
FIG. 27 shows the bearing 500 provided with a rotation sensor having the aforementioned structure in a state assembled into a motor. FIG. 27 is a sectional view showing the structure of the motor into which the bearing 500 provided with a rotation sensor is assembled. A motor rotor 11 assembled into a rotary shaft 12 is supported in a housing 13 by a front bearing 14 and a rear bearing 15, and a motor stator 10 is also fixed to the housing 13. In the motor shown in FIG. 27, the rear bearing 15 stores a rotation sensor.
When a large current is fed to the motor stator 10, the flow of a magnetic flux cannot be ignored. A magnetic loop is generated to pass through the motor rotor 11, the rotary shaft 12, the inner ring 3, the outer ring 1 and the housing 13 and return to the motor stator 10 as shown by arrows in FIG. 27. At this time, a nonmagnetic part occupies most part of the space between the inner ring 3 and the outer ring 1, except the rolling elements 2. The magnetic rolling elements 2 are in point contact with the inner ring 3 and the outer ring 1, and arranged only on about six portions of a circumference. Therefore, a path through the inner ring 3, the rolling elements 2 and the outer ring 1 has high magnetic resistance.
Consequently, the bearing 15 exhibits high magnetic resistance, readily leading to leakage of a magnetic flux. The leaking magnetic flux flows to the sensor case fixing ring 6 and the mandrel 5, which are magnetic members, to disadvantageously exert bad influence on the magnetic sensor 8 and disturb a sensor signal.
(Second Prior Art)
Another type of bearing provided with a rotation sensor has a rotating element provided with a sensor target such as a magnetic pattern and a fixed element provided with a sensor element for detecting relative rotational movement of the sensor target with respect to the sensor element and outputting an electric signal.
FIGS. 28 and 29 show the sectional structures of bearings 600a and 600b provided with rotation sensors according to second prior art. Each of the bearings 600a and 600b provided with rotation sensors has an inner ring 601, an outer ring 603 and rolling elements 602 provided in an annular space defined between the inner ring 601 and the outer ring 603. When the inner ring 601 is employed as a rotating element, an encoder ring 604 serving as a sensor target is fixed to the inner ring 601. When the outer ring 603 is employed as a non-rotating element, a rotation detecting sensor 605 detecting rotation of the encoder ring 604 is fixed to the outer ring 603.
(Problem of Second Prior Art)
In order to extract an output signal from the rotation detecting sensor 605, a cable must be extracted from a circuit board into which the rotation detecting sensor 605 is assembled. When the outer diameter of the bearing 600a or 600b is larger than 30 mm, a cable 610 can be extracted from an axial end surface of the bearing 600a provided with a rotation sensor as shown in FIG. 28 or from the outer peripheral surface of the bearing 600b provided with a rotation sensor as shown in FIG. 29.
If the outer diameter of the bearing 600a or 600b is smaller than 30 mm, however, no space for extracting the cable 610 is defined but it is difficult to extract a signal from the rotation detecting sensor 605.
(Third Prior Art)
FIG. 30 is a sectional view showing a bearing provided with a rotation sensor according to third prior art. Referring to FIG. 30, this bearing provided with a rotation sensor is an antifriction bearing formed by an outer ring 701, an inner ring 703 and rolling elements 702, and a pulser ring 704 is fixed to the rotating element (the inner ring 703, for example) while a magnetic sensor 705 is fixed to the non-rotating element (the outer ring 701, for example) through a sensor case 706. A magnetic encoder is formed on the surface of the pulser ring 704. Such a bearing provided with a rotation sensor, which is miniature and strong with no requirement for assembly control, is utilized for supporting a motor or the like.
Alternatively, the outer ring 701 and the inner ring 703 may be employed as a rotating element and a non-rotating element respectively.
The sensor of such a bearing provided with a rotation sensor generates an analog output shown in FIG. 31A or a rectangular wave output shown in FIG. 31B. An analog output type sensor must have repetitive reproducibility of a sinusoidal waveform, and hence uniformity of magnetization intensity is important for the magnetic encoder. A rectangular wave output type sensor utilizes an output signal in a saturated waveform, and hence large magnetization intensity is more strongly required as compared with uniformity of the magnetization intensity. When the magnetization intensity is large, magnetic field strength steeply changes to advantageously improve pitch accuracy or increase a sensor gap.
(Problem of Third Prior Art)
In general, anisotropic magnetic powder is employed for the magnetic encoder regardless of the output signal. When anisotropic magnetic powder is employed for an analog output type encoder, however, the amplitude of a sinusoidal wave output is disadvantageously largely dispersed.
(Fourth Prior Art)
FIG. 32 is a longitudinal sectional view of a bearing provided with a rotation sensor according to fourth prior art. Referring to FIG. 32, the bearing provided with a rotation sensor is an antifriction bearing formed by an outer ring 801, an inner ring 803 and rolling elements 802, and a pulser ring 804 is fixed to a rotating side (the side of the inner ring 803, for example) through a mandrel 805 while a magnetic sensor 808 is fixed to a non-rotating side (the side of the outer ring 801, for example) through a sensor case 807 and a sensor case fixing ring 806. Such a bearing provided with a rotation sensor, which is miniature and strong with no requirement for assembly control, is employed as a bearing for supporting a motor or the like.
(Problem of Fourth Prior Art)
When the bearing provided with a rotation sensor shown in FIG. 32 is assembled into the magnetic path of a coil or a magnet generating a large magnetic field, however, an output of the bearing provided with a rotation sensor may malfunction by a leakage flux caused by the external magnetic field.
FIG. 33 shows the bearing provided with a rotation sensor assembled into a motor, for example. Referring to FIG. 33, a front bearing 814 and a rear bearing 815 provided with a rotation sensor support a motor rotor 811 assembled into a rotary shaft 812 in a housing 813, to which a motor stator 810 is fixed. When a large current is fed to the motor stator 810, the flow of a magnetic flux cannot be ignored but a magnetic loop is generated to pass through the motor rotor 811, the rotary shaft 812, the inner ring 803, the outer ring 801 and the housing 813 and return to the motor stator 810 as shown by arrows in FIG. 33.
When the direction of the current is reversed, the magnetic loop is also reversed. At this time, a nonmagnetic part dominantly occupies the space between the inner ring 803 and the outer ring 801 except the rolling elements 802 and a retainer 819 and to increase magnetic resistance, and hence a magnetic flux readily leaks to influence the magnetic sensor 808 and disturb a sensor signal or cause a malfunction.